The development of B cells from the earliest committed precursors to mature secreting cells is marked, phenotypically and genotypically, by the sequential expression of a series of cell surface antigens and immunoglobulin gene rearrangements. One of the earliest markers expressed in the B cell and other hematopoietic lineages is the Ly-17 alloantigen. Phenotypic, functional and biochemical studies of antibodies to allelic specificities of Ly-17 showed that they recognize genetically-determined polymorphisms of the murine Ig gamma Fc receptor. Analyses of B lineage lymphomas indicated that B cell development does not involve a single linear pathway of differentiation. Although all B cells may originate from a common precursor population, they appear to rapidly diverge into independent Ly-1+ and Ly-1- pathways. A precursor cell for both these pathways may express the Mac-1 antigen indicating that the B cell and myeloid differentiative pathways are closer than appreciated previously. Altered expression of the myc oncogene is associated with development of B-lineage neoplasms in mouse and man whereas expression of the raf oncogene is associated with development of sarcomas and erythroblastosis in mice. Mice infected with raf/myc - containing viruses developed not only sarcomas and erythroblastosis but pre-B, B and T cell lymphomas and pancreatic adenocarcinoma. The non-raf-like neoplasms were also induced with retroviruses expressing myc alone.